1. Field of the Invention
The present invention relates to an apparatus and method for imaging an optical coherence tomographic image and, more particularly, to an imaging apparatus and method for use in an ophthalmic field.
2. Description of the Related Art
Currently, various ophthalmic apparatuses using optical devices are used. Such apparatuses are, e.g., an anterior eye camera, a fundus camera, and a confocal scanning laser ophthalmoscope (scanning laser ophthalmoscope (SLO)). Particularly, an optical coherence tomographic imaging apparatus (hereinafter referred to as an OCT apparatus) is configured to obtain a tomographic image of an inspection target at high resolution, and is becoming an indispensable apparatus in clinics dedicated to retinal outpatients. The above OCT apparatus uses a low-coherence light-source as a source of light. Light from the light-source is split into measurement light and reference light through a split optical path such as a beam splitter. With one of such split-light components, i.e., the measurement light, an inspection target such as an eye via a measurement optical path is irradiated. Return light thereof is guided to a detection position via a detection optical path. The return light is reflected light or scattered light, which contains information concerning an interface in a direction of light-irradiating of the inspection target. The other light component, i.e., the reference light is reflected by a reference mirror or the like via a reference optical path, and guided to the detection position. The return light and the reference light are caused to interfere with each other. Interference light generated therebetween is guided to a photoelectric conversion element such as a charge-coupled device (CCD) line sensor or a complementary metal-oxide semiconductor (CMOS) line sensor via an optical element such as a spectroscope. Then, wavelength spectra are collectively output therefrom as electric signals. The electric signals output in the above manner are converted by an analog-to-digital (A/D) converter into digital signals. In addition, the digital signals obtained by A/D conversion are subjected to a Fourier transform. Consequently, a tomographic image of the inspection target can be obtained. This method is referred to as a spectrum domain (hereinafter referred to as SD) method.
If a retina is employed as the inspection target in the OCT apparatus, a three-dimensional (3D) image of the retina can be acquired by scanning measurement light on the retina using a Galvano mirror or the like. However, if image information is acquired by setting, as the inspection target, a part of an eyeball such as a retina, it is difficult to accurately acquire image information concerning the eyeball, because of a subject's involuntary eye movement during visual-fixation. Thus, Japanese Patent Application Laid-Open No. 2008-104628 proposes an apparatus for imaging a conjunctiva and a sclera of a subject's eye by excluding effects of the subject's involuntary eye movement during visual-fixation.
The apparatus proposed in Japanese Patent Application Laid-Open No. 2008-104628 employs a system configured to exclude effects of the subject's involuntary eye movement during visual-fixation according to a method of adjusting a position of an imaging area corresponding to each taken image by following the subject's involuntary eye movement during visual-fixation, using a camera capable of imaging a target at a rate of 200 frames per second (fps). However, in order to acquire a 3D-image of an ocular-fundus, according to the system discussed in Japanese Patent Application Laid-Open No. 2008-104628, hundreds milliseconds (ms) are taken to acquire a single 3D-image. That is, about only two 3D-images can be acquired per second, as will be described below. Thus, according to the system discussed in Japanese Patent Application Laid-Open No. 2008-104628, it is difficult to eliminate effects of the subject's involuntary eye movement upon an image during visual-fixation, particularly, effects of a flick thereupon causing a largest displacement of a retina among the motions of the subject's involuntary eye movement during visual-fixation.
Hereinafter, these problems are described in detail. FIGS. 2A through 2C illustrate aspects of a subject's involuntary eye movement during visual-fixation. In FIG. 2A, a solid curve 201 represents a trajectory of the center of the retina of the subject's eye. As illustrated in FIG. 2A, the retinal center initially positioned at point A moves to point B through paths that are represented by two-headed dashed arrows 202, 203, and 204 and generated by the subject's involuntary eye movement during visual-fixation. Usually, the involuntary eye movement includes a combination of the following three motions, i.e., a drift 202 in which the retinal center moves at low speed, a tremor 203 in which the retinal center repeats fine zigzag movements, and a flick 204 in which the retinal center instantaneously moves. In order to describe features of the subject's involuntary eye movement during visual-fixation, FIG. 2B illustrates the movement of the retinal center, which has been illustrated in FIG. 2A, by setting an abscissa axis and an ordinate axis to represent time, and acceleration in a direction of a y-axis (vertical direction) of a position of the retinal center, respectively. The retina operates in two-dimensional directions, i.e., a horizontal direction and a vertical direction, respectively, due to the involuntary eye movement during visual-fixation. For brevity of description, only an operation in the vertical direction is described. As illustrated in FIG. 2B, accelerations 212 are obtained where a drift is performed. Accelerations 213 are obtained where a tremor is performed. Accelerations 214 are obtained where a flick is performed. FIG. 2C illustrates the features of the three motions included in the involuntary eye movement during visual-fixation.
The drift is a small motion that substantially always occurs and that causes a retinal displacement of 7 μm through 17 μm in a time-period of 0.2 seconds (s) through 1 s. The tremor is a slight quiver that occurs periodically every period corresponding to a frequency of 30 Hertzes (Hz) through 100 Hz, i.e., every period of 10 ms through 33 ms and that causes a retinal displacement of 0.3 μm through 3.5 μm. The flick is an abrupt motion that occurs periodically every period corresponding to a frequency of 0.2 Hz through 2 Hz, i.e., every period of 500 ms through 5 s and that causes a retinal displacement of 7 μm through 52 μm in a time-period of 10 ms through 30 ms.
Hereinafter, it is described a case of acquiring a 3D-image of an ocular-fundus by setting a scanning area of 6 millimeters (mm)×6 mm and a resolution corresponding to a size of 20 μm×20 μm, using the apparatus discussed in Japanese Patent Application Laid-Open No. 2008-104628. It is assumed that interference light dispersed by a spectroscope is subjected to photoelectric conversion using a high-speed line camera at a line rate of 250 kilo-Hz (kHz), and that 20% of a time-period in which the ocular-fundus is two-dimensionally scanned using a Galvano mirror, is unavailable for reading a 3D-image of the ocular-fundus. In this case, time T required to read the 3D-image is given by:T=(6/(20*10−3))2/(250*103)/0.8=450 ms.When a 3D-image of the ocular-fundus is acquired on the above conditions, among the motions of the involuntary eye movement during visual-fixation, the drift and the tremor are not so problematic in acquiring a 3D-image, in view of the resolution and the displacement per unit time. However, if a flick occurs in a time-period of 450 ms, which is required to acquire a 3D-image, a position of an eye is extremely displaced in a short time. Accordingly, continuity of a read image cannot be assured. Consequently, an accurate 3D-image thereof cannot be acquired. According to the method using a camera capable of imaging at a rate of 200 fps, it is difficult to eliminate effects of the involuntary eye movement upon an image during visual-fixation, particularly, effects of a flick thereupon causing a largest displacement of a retina among the motions of the subject's involuntary eye movement during visual-fixation.